MAGNETIC
It is quite well known to record various types of information, either analog or digital, on apparatus employing ferromagnetic coatings on structures in a variety of forms such as tape, disks, drums, and the like. In these structures a ferromagnetic coating is applied as a thin film on a non-ferromagnetic substrate. A broad variety of such magnetic coatings have been developed and used. The magnetic characteristics of the coating determine the type and amount of information of a given type which may be magnetically recorded thereon.
Digital computers handle large amounts of information at high speeds and this information is coded in binary form which may be recorded on a ferromagnetic coating on a disk, tape, or drum. Each individual piece of binary information in a digital computer is identified as a "bit" and these bits are generally recorded in tracks which are produced by the passage of a magnetic recording head over the surface of the coating. A large number of tracks may be arranged parallel to each other on the recording medium. The tracks are not physical structures isolated from each other but are identifiable magnetic regions in a continuous film. The number of bits per square inch which may be recorded on a ferromagnetic coating is an important factor in determining the size and cost of recording equipment. The trend for several years has been towards higher and higher density recording.
Since, in the design of magnetic recording equipment, there are mechanical considerations not associated with the characteristics of the ferromagnetic coating which place minimal limitations on the width of each recording track and the distance between tracks it has become customary to specify the recording density as the number of bits recorded per linear inch of track, commonly stated as "bits per inch".
The "bit length" (inches per bit) is the reciprocal of the bit density in a magnetic recording film. Transfer rate refers to the number of bits of information transferred to or from the magnetic recording medium by the recording apparatus. Typically the units are thousands or millions of bits per second. The bit period (seconds per bit) is the reciprocal of the transfer rate. Thus, for example, in a system recording at a bit density of 10,000 bits per inch and at a transfer rate of 10 megabits per second, the bit length is 100 microinches and the bit period is 100 nanoseconds.
The magnetic characteristics of a ferromagnetic coating for recording digital information are highly important in determining the maximum number of bits per inch which may be recorded and intelligibly reproduced or "read". The recording density is markedly influenced by the form of the hysteresis loop which is characteristic of the coating as well as by the coercivity and retentivity of the coating.
FIG. 1 illustrates the general form of the hysteresis loop of a ferromagnetic material such as is useful in thin film recording devices. The hysteresis loop is a plot of the directed intensity of magnetization B of the material as a function of the directed magnitude of the external magnetizing field H. Initially, before being subjected to a magnetizing influence, a ferromagnetic film or the like has an inherent magnetization very near or at the origin of the graph of FIG. 1. After being subjected to magnetization the material will have a residual magnetization at any point on or within the hysteresis loop depending on its previous magnetic history. In recording devices it is usual that the intensity of external magnetizing is sufficient to saturate the magnetic material and the magnetization B lies on the line forming the loop.
Assume that because of prior magnetic history the initial state of magnetization of a ferromagnetic material is B.sub.r .sup.-. That is when the external magnetizing field H is zero, the ferromagnetic film has a residual intensity of magnetization of B.sub.r .sup.-. As the applied magnetic field H increases in the H.sup.+ direction to a value equal to or greater than H.sub.s .sup.+ the value of B will vary in accordance with the lower curve to a value of B.sub.s .sup.+. The field intensity H.sub.s is the saturation field at which all of the magnetic domains which can be oriented by the field have been so oriented. Further increase in the value of H will not result in any increase in the value of B. When the value of the magnetic field is reduced to zero after having attained saturation H.sub.s .sup.+ the value of B in the ferromagnetic material varies in accordance with the upper curve to B.sub.r .sup.+. That is, when the magnetic field is completely removed there is a residual intensity of magnetization in the ferromagnetic material of B.sub.r .sup.+.
When the field intensity of magnetization is increased in the opposite direction, that is, towards H.sup.-, the intensity of magnetization B in the ferromagnetic material follows along the upper curve until a saturation value of B.sub.s .sup.- is reached and further increases in magnetic field intensity will not increase the magnetization of the material. When the magnetic field is again reduced to zero the intensity of magnetization in the ferromagnetic materials follows the lower curve in FIG. 1 to return to the initial state of magnetization B.sub.r .sup.-.
The hysteresis loop is symmetrical about the origin and the signs of B and H are arbitrary. The value of B.sub.r when field H is zero is known as retentivity or maximum remanence and has units of Gauss. The value of field H.sub.c needed to bring magnetization to zero is known as coercivity and the units are Oersteds.
The value obtained by dividing the residual intensity of magnetization or retentivity B.sub.r by the saturation magnetic intensity B.sub.s is a measure of how well the magnetic domains in the ferromagnetic material remain oriented when the magnetizing field is reduced to zero after equalling or exceeding the saturation value H.sub.s. This value is commonly known as the "squareness ratio" of the hysteresis loop. Generally speaking in a magnetic recording film it is desirable to have a high squareness which indicates that there is a relatively strong residual magnetization in the film after recording. Squareness ratios in the order of about 0.75 to 0.90 have been considered acceptable for thin film magnetic recording devices.
Characteristics of the hysteresis loop other than squareness, coercivity and retentivity are involved in determination of the suitability of a ferromagnetic coating for use as a digital recording medium at very high bit densities. Recording densities of 5000 bits per inch are employed in some commercially available equipment and densities exceeding 10,000 bits per inch may be practical. Transfer rates of millions of bits per second may be used in disk memories.
Referring again to FIG. 1 it will be noted that the variation of B with H is non-linear as H is increased from zero to saturation H.sub.s. The great majority of the magnetic domains are switched as H increases from H.sub.1 to H.sub.2. As the angles .alpha. and .beta. as shown in FIG. 1 approach 90.degree. and the instep radius R.sub.1 and knee radius R.sub.2 approach zero, the range of values of the magnetizing field H over which the magnetic state of the ferromagnetic coating is appreciably affected steadily decreases. The limit of this variation is the perfectly rectangular hysteresis loop illustrated in FIG. 2. As pointed out hereafter the closer one can approach such a rectangular loop the better the recording film is for recording high bit densities. To avoid confusion with the commonly employed term "squareness" this characteristic of hysteresis loops will be referred to herein as "rectangularity". So far as is known, this term has not previously been employed with respect to magnetic hysteresis loops.
In one type of system for recording digital information known as the non-return-to-zero (NRZ) saturation mode, "ones" are recorded by applying a magnetizing field having an intensity greater than H.sub.s in one direction and "zeros" are recorded by applying a field of equal intensity in the opposite direction. That is, on the recording medium, the bits having a value of 1 are represented by an intensity of magnetization of B.sub.r .sup.+ and the bits having a value of 0 are represented by a state of magnetization of B.sub.r .sup.-.
A consecutive series of ones or zeros is recorded by maintaining the saturation field of the recording head in the appropriate direction for the required number of bit periods. When the value being recorded changes from zero to one or vice versa, the magnetic polarization of the recording head is reversed, thereby reversing the magnetic polarization of the ferromagnetic coating. This reversal of polarization is called a transition and the length of the transition along the recording track exerts a pronounced influence on the form of the electrical signal reproduced by the reading head as it passes over the transition.
In modern digital recording apparatus the relative velocity between the recording head and the ferromagnetic coating may be as great as several thousand inches per second. The reading heads, of course, have similar relative velocities. Such velocities may be obtained, for example, by a stationary head and a rapidly rotating disk or drum. Reversal of the magnetizing field produced by the recording head is accomplished by reversing the direction of flow of an electric current through the winding of the head. This reversal of polarity requires a small but finite period of time. During this short period successive increments of coating along the recording track are subjected to magnetic fields of every value between the saturation magnetizing field H.sub.s in one direction and the saturation field H.sub.s in the other direction. It will be apparent that as the rectangularity of the hysteresis loop increases, the range of magnetizing field intensities which appreciably affect the magnetic state of recording film decreases. This, of course, reduces the transition length and with a perfectly rectangular hysteresis loop the transition length will be governed solely by the rates of current change possible in the electronic equipment and recording head. Reduction of the transition length that must be accounted for between adjacent bits makes available more space for the bit information itself and short transition lengths are highly important for high bit densities.
As the reading head passes over the magnetic transition on the recording film the magnetic gap in the head cuts the magnetic flux field emitted by the transition, thereby inducing an electric current in the winding of the rear head. The electric signal generated in the head is proportional to the first derivative of the flux density as a function of time, d .PHI./dt. That is, as the length of the transition is reduced the resultant signal amplitude will increase. Thus, for a given signal output the energy product, retentivity times coercivity (B.sub.r .times. H.sub.c), can safely be decreased as the rectangularity of the hysteresis loop is increased. Generally it has been considered desirable to have the area of the hysteresis loop relatively large, that is, have a large energy product so that the magnetic recording medium retains more energy. The retention of more energy implies that more energy is applied in the course of reversing the polarity of magnetization in the film. This, of course, requires application of more energy to the recording head. Increasing rectangularity of the hysteresis loop with resultant higher signal amplitude minimizes the desirability of a large energy product, thereby relaxing the energy requirements of the electronics and recording heads. A substantial energy product is, of course, still desirable to minimize the influence of stray demagnetizing effects.
The many transitions within a recording medium induce a general demagnetizing field which tends to increase the transition length. It is generally recognized that the transition length tends to be increased by increasing retentivity B.sub.r and decreased by increasing coercivity H.sub.c. An increase in coercivity requires a greater magnetizing field to saturate the ferromagnetic recording medium and hence complicates the problem of designing recording heads and electronics for operation at high transfer rates. However, the decrease in energy product which becomes permissible with increased rectangularity of the hysteresis loop permits a decrease in the retentivity of the coating. Increased rectangularity of the hysteresis loop provides an additional advantage in the design of recording systems. With increasing rectangularity the saturation field H.sub.s approaches the coercivity H.sub.c and therefore a recording head which provides a given magnetizing field can record on a coating having greater coercivity.
An important property of a magnetic recording film is the attenuation ratio at high bit densities. The attenuation ratio as used herein is the ratio of the readback signal amplitude at low bit density divided by the readback signal amplitude at high bit density. Such a measurement is made, for example, by recording a train of signals at a bit density of 800 bits per inch. The train of signals is then read in a conventional manner and the signal amplitude measured. The same procedure is followed by recording data on the medium at a bit density of say 5000 bits per inch and then measuring the readback signal amplitude as this data is read. Losses of signal amplitude due to either or both the recording or reading processes show up as an increase in the attenuation ratio over the optimum value of 1 which indicates that the film is as suitable for high bit densities as it is for low.
Processes for forming ferromagnetic coatings for use as recording media in apparatus employing high bit densities and high transfer rates should provide coatings which exhibit hysteresis loops having a high order of rectangularity. That is, the instep radius R.sub.1 and the knee radius R.sub.2 should both be small and the angle .alpha. (FIG. 1) should approach 90.degree.. Recording heads and the electronics for magnetic recording systems are designed for use with recording media having predetermined values of retentivity and coercivity. It is therefore highly desirable to be able to predict these magnetic properties of a coating. It is desirable to control the coercivity and retentivity of the magnetic coating material over a broad range of values for design and use of systems having a variety of applications. Preferably the coercivity and retentivity should be independently controllable so that coatings can be provided with any desired properties.